This invention relates generally to cardiac rhythm management systems, and particularly, but not by way of limitation, to a system for detecting atrial fibrillation and for discriminating atrial fibrillation from atrial flutter.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. The body""s autonomic nervous system regulates intrinsic electrical heart activity signals that are conducted to atrial and ventricular heart chambers on the left and right sides of the heart. The electrical heart activity signals trigger resulting heart contractions that pump blood. However, some people have irregular cardiac rhythms, referred to as arrhythmias. Some of the most common arrhythmias are atrial fibrillation (AF) and atrial flutter (AFL). Atrial fibrillation can result in significant patient discomfort and even death because of number of associated problems, including: (1) an irregular heart rate which causes the patient discomfort and anxiety, (2) loss of synchronous atrioventricular contractions which interferes with cardiac hemodynamics, resulting in varying levels of congestive heart failure, and (3) stasis of blood flow, which increases the vulnerability to thromboembolism. AF most commonly exhibit heartbeat rates of about 400 to 600 per minute in humans. On the other hand AFL is characterized by approximately 250 to 300 beats per minute. AFL is thought to result from a counterclockwise reentry circuit in the right atrium associated with the atrial septum and the right atrial freewall. The reentry circuit normally travels between the inferior vena cava and the tricuspid valve. Overlap between the ranges of number of beats per minute in AF and AFL is not uncommon.
One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of cardiac rhythm management system. Such a system may be implanted in a patient to deliver therapy to their heart.
Cardiac rhythm management systems include, among other things, implanted rhythm management devices. Implanted rhythm management devices deliver, among other things, timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous lead wire or catheter (referred to as a xe2x80x9cleadxe2x80x9d) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as xe2x80x9ccapturingxe2x80x9d the paced heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Such devices are often used to treat patient""s hearts exhibiting arrhythmias. Implanted rhythm management devices is also used to deliver high-energy defibrillation pulses via lead wire having one or more electrodes disposed in or about the heart for providing defibrillation therapy.
Implanted rhythm management devices generally include sensing circuits to sense electrical signals from a heart tissue in contact with the electrodes. Then a controller in the implanted rhythm management device processes these signals and issues command signals to therapy circuits, for delivery of electrical energy such as pacing and/or defibrillation pulses to the appropriate electrodes in or about the heart to provide therapy to the heart. The controller may include a microprocessor or other controller for execution of software and/or firmware instructions. The software of controller may be modified to provide different parameters, modes, and/or functions for the implantable device to adapt or improve performance of the device. Generally algorithms are used in software and/or firmware residing in the controller to discriminate sensed signals and to provide appropriate therapy to the heart. Algorithms are also used to discriminate AF from AFL. In general, AF exhibits shorter cycle lengths (CLs) and greater cycle length (CL) variability than AFL. Current techniques are based on interval information and ignore serial interval relationships. Thus, there is a need for a more reliable, more sensitive and less computationally oriented method of detection of AF and discrimination from AFL in implanted rhythm management devices to provide the appropriate therapy to the heart and to reduce patient morbidity and discomfort.
The above-mentioned shortcomings, disadvantages and problems are addressed by the present invention, which will be understood by reading and studying the following specification. The present system provides, among other things, a reliable technique for discriminating atrial fibrillation (AF) from atrial flutter (AFL). The present technique allows for reduced computation and increased sensitivity and specificity in discriminating between AF and AFL.
According to one aspect of the present subject matter, a sensor disposed in or about a heart, senses a cardiac signal. A controller through a sensing circuit then receives the sensed cardiac signal. The controller processes the cardiac signal to discriminate AF from AFL, by computing average cycle length-to-cycle length variability (high frequency variability of cycle lengths) between adjacent cycle lengths for a pre-determined number of sequential cycle lengths, and comparing the computed high frequency variability, with one or more pre-determined threshold values, to discriminate AF from AFL, and issue a command signal to a therapy circuit based on the outcome of the comparison. Then the controller provides the appropriate therapy to the heart through an electrode disposed in or about the heart. As a result of using such a sequence-based algorithm, the system is generally capable of providing a superior performance over existing algorithms in discriminating AF from AFL, which neglect any serial cycle length properties. Current algorithms based on interval techniques rely on sequence-independent measures of atrial interval dispersion (e.g., standard deviation, range, local extrema, etc.,) to differentiate the relatively fixed cycle length of AFL from the relatively large cycle-to-cycle variability characteristic of AF. Viewed in the interval domain, AFL presents a constant or slowly changing (low frequency) cycle length, while AF exhibits large intercycle duration variations (high frequency activity). The present invention consists of a measure of average cycle length-to-cycle length variability (in the interval sense) for a given interval sequence. With the substantial difference in high frequency activity between AF and AFL (despite similar average cycle lengths), the sequence based measure of average high frequency variability will prove to be a highly specific discriminator when robust atrial interval sequences are available. The algorithm may also prove useful in discriminating certain ventricular tachycardias from ventricular fibrillation. In one embodiment, the comparator compares the average cycle length with the average cycle length-to-cycle length variation, and issues a command signal based on the outcome of the comparison. In another embodiment the comparator compares the central tendency (e.g., mean, median or mode) cycle length-to-cycle length variation, and issues a command signal based on the outcome of the comparison. In one embodiment the comparator further classifies the computed average cycle length-to-cycle length variation to detect a cardiac arrhythmia.
In one embodiment, the electrode is disposed in and/or around an atrial region of a heart to sense a cardiac signal around an atrial chamber. In one embodiment, the electrode is disposed in and/or around a ventricular region of the heart to sense the cardiac signal. In another embodiment, a therapy includes providing pacing pulse electrical energy, when AFL is detected by the controller. In another embodiment, the therapy includes providing defibrillation pulse electrical energy when AF is detected by the controller. In another embodiment, the therapy includes activating an implanted or external device to administer a drug therapy when controller detects AF or AFL. In another embodiment, an external programmer, remote from an implanted cardiac rhythm management system, is used to communicate with the controller and to program the controller. In one embodiment a timer is included to introduce a delay between receiving the command signal from the comparator and administering the drug therapy to the heart. Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.